Planar microstrip antenna elements and arrays are utilized in a variety of applications due to their simple structure, packaging advantages, and ease in fabrication and integration with associated electronic circuitry. However, planar microstrip antennas are inherently limited in input impedance bandwidth, which is a significant disadvantage in variable and wideband frequency applications, and particularly in spread-spectrum applications.
It is known that the input impedance bandwidth of planar microstrip antenna elements and arrays can be improved by aperture feeding the radiating elements. This can be accomplished by constructing the antenna element or array as a set of three vertically aligned metal layers separated by intervening dielectric layers. The center metal layer is used as the ground plane and the two outer metal layers are respectively etched to form a feed structure and one or more radiating patches, with energy being coupled from the feed structure to the radiating patches through corresponding apertures etched in the ground plane layer. It is also known that the bandwidth can be further enhanced, at least in the case of rectangular radiating patches, through the addition of rectangular parasitic metal strips at the non-resonant edges of the radiating patches. The parasitic strips are co-planar with the radiating patches and capacitively load the respective radiating patches to make their electrical impedance more uniform across the range of activation frequency. However, antenna elements incorporating these features are still bandwidth limited and tend to exhibit excessive off-boresight variation in beam directivity. Accordingly, what is needed is a linearly polarized planar microstrip antenna having both improved input impedance bandwidth and off-boresight radiation uniformity.